The present invention relates to electrophoretic displays, especially encapsulated electrophoretic displays, and to systems for addressing such displays.
There are a number of interesting display media which provide good optical appearance, the ability to be constructed in large areas or on flexible substrates, low cost, and ease of manufacture. Such display media include microencapsulated electrophoretic displays, rotating bichromal ball displays, suspended particle displays, and composites of liquid crystals with polymers, including polymer dispersed liquid crystals, polymer stabilized liquid crystals, and liquid crystal gels.
One drawback of such displays is that they are difficult to practically and economically address. One common means of addressing is known as direct drive addressing, in which each pixel is controlled by its own external drive circuit. This scheme is both expensive and impractical for displays containing a large number of pixels and for displays containing pixels that are tightly packed.
Another means of addressing is active matrix drive addressing, in which an electrically non-linear element is deposited on the display substrate. Examples of such electronically non-linear elements include transistors, diodes, and varistors. While this type of addressing is well-known and widely practiced, it is expensive to produce and difficult to achieve on plastic substrates.
A third means of addressing uses multiplexing, in which the conductive portions of the substrate are patterned so that rows of pixels on the substrate are electrically connected and columns of pixels on the substrate are also electrically connected. Typically, voltages are sequentially placed on the row electrodes, with the pixel data for each row being placed on the column electrode. This type of addressing is used for a variety of display media. Its use is limited, however, to displays in which the optical response as a function of applied voltage is non-linear and in which there is a significant voltage threshold to turn on the pixels. Display media which do not show a pronounced voltage threshold show poor contrast when driven with multiplex addressing drive schemes.
The purpose of this disclosure is to describe electrophoretic displays, especially encapsulated electrophoretic displays, and systems for addressing display media of such displays. Systems of the invention allow for the addressing of display media that have poor threshold behavior without the high costs associated with using direct drive and active matrix drive addressing schemes. This is accomplished by using a multiplex addressing drive scheme in conjunction with an emissive material that does possess a pronounced voltage threshold.
The present invention provides electrophoretic displays, especially encapsulated electrophoretic displays, and systems for addressing such displays. Displays of the invention include an organic, light-emitting layer, a photoconductive layer, and an electrophoretic layer. Such displays may be rigid or flexible. Displays of the invention may also include a reflective substrate to direct light from the organic, light-emitting layer to the photoconductive layer. Displays of the invention may also include a dielectrophoretic layer, which is preferably fenestrated. Finally, displays of the invention may also include a capacitor.
In one embodiment, the invention relates to an electrophoretic display including an organic, light-emitting layer, a photoconductive layer adjacent the organic, light-emitting layer, and an electrophoretic layer adjacent the photoconductive layer. Light from the organic, light-emitting layer strikes the photoconductive layer at a first point on a first side of the photoconductive layer, which faces the organic, light-emitting layer. A voltage is then generated at a second point on a second side of the photoconductive layer. This second point corresponds to the first point and faces the electrophoretic layer. The voltage at the second point addresses the electrophoretic layer at a predetermined point on the electrophoretic layer.
When the display is not illuminated, the impedance of the photoconductive layer is much greater than the impedance of the electrophoretic layer. The photoconductive layer therefore drops the majority of the applied voltage. When the display is illuminated, the impedance of the photoconductive layer decreases, and the majority of the applied voltage then drops across the electrophoretic layer, forming an image. Specifically, the photoconductive layer is biased at a voltage on the xe2x80x9crearxe2x80x9d side, which faces the organic, light-emitting layer. The portions of the photoconductive layer that are exposed to light effectively transfer the voltage to the xe2x80x9cfrontxe2x80x9d side of the photoconductive layer, which faces the electrophoretic layer. Depending on the ratios of the capacitances and the resistances, the reset pulse may require a slow ramp to avoid the capacitive regime.
In another embodiment of the invention, an emissive display includes an organic, light-emitting layer and a photoconductive layer disposed under the organic, light-emitting layer. In this embodiment, the organic, light-emitting layer is addressable at a first predetermined voltage. A first fraction of this first predetermined voltage drops across the organic, light-emitting layer, and a second fraction of this first predetermined voltage drops across the photoconductive layer. When the organic, light-emitting layer is addressed using this first predetermined voltage, it emits light, which strikes the photoconductive layer. This light causes the impedance of the photoconductive layer to decrease, so that the fraction of the first predetermined voltage dropping across the photoconductive layer is decreased and the fraction of the first predetermined voltage dropping across the organic, light-emitting layer is increased. The organic, light-emitting layer may then be addressed at a second predetermined voltage, which is lower than the first predetermined voltage. In an alternative embodiment, the emissive display includes a fenestrated dielectrophoretic layer, which modulates the amount of light striking the photoconductive layer.
The organic, light-emitting layer for use in displays of the invention includes an organic material disposed on a clear substrate. The clear substrate may be a glass, a plastic, or a polyester substrate, for example. The organic, light-emitting material may be an organic compound, an organometallic compound, an oligomer, or a polymer. Dispersed within the organic material may be inorganic semiconductors, such as CdSe conductors, for example.
The photoconductive layer for use in displays of the invention includes a photoconductive material, such as 2,4,7-trinitro-9-fluorenone complexed with poly(N-vinylcarbazole). The photoconductive material may be an organic photoconductive polymer, a dye-aggregate photoreceptor, or a pigment-based photoreceptor. In one embodiment, the photoconductive layer is disposed on a clear substrate, such as a glass, a plastic, or a polyester substrate, for example. In one embodiment, an optical barrier layer is disposed over or adjacent to the photoconductive layer. The optical barrier layer is a dispersion of opaque conductive particles in a polymer matrix, such as a dispersion of black pigment particles in an epoxy binder, for example. In other embodiments, the photoconductive layer includes a first photoconductive material and a second photoconductive material. The second photoconductive material is sensitive to a different variable of light than the first photoconductive material. The variable of light may be the wavelength of the light, the intensity of the light, or the duration of the light.
The electrophoretic layer for use in displays of the invention may be an encapsulated electrophoretic layer or a dielectrophoretic layer. An encapsulated electrophoretic layer of the invention includes a plurality of particles dispersed in a suspending fluid, which is encapsulated in a polymer matrix. The polymer matrix may include an aqueous polymer latex, such as a polyurethane, for example. The polymer matrix may be coated onto a substrate, such as a glass, plastic, or polyester substrate, for example.
In another embodiment of the invention, a display includes a clear top electrode. This clear top electrode may comprise a conductive material on a substrate. The clear top electrode may be indium tin oxide (ITO) coated onto a glass, plastic, or polyester substrate, for example.
The invention will be understood further upon consideration of the following drawings, description, and claims.